Unveiling Strong\nThin Film Confinement Effects on\nSemirigid Conjugated Polymers

Nanoconfinement has\nbeen recognized to induce significant\nchanges\nin the physical properties of polymeric films when their thickness\nis less than 100 nm. Despite extensive research on the effect of nanoconfinement\non nonconjugated polymers, studies focusing on the confinement effects\non dynamics and associated electronic and mechanical properties for\nsemiconductive and semirigid conjugated polymers remain limited. In\nthis study, we conducted a comprehensive investigation into the nanoconfinement\neffects on both p- and n-type conjugated polymers having varying chain\nrigidity under different degrees of confinement. Using the flash differential\nscanning calorimetry technique, it was found that the increased molecular\nmobility with decreasing film thickness, as indicated by the depression\nof glass transition temperature (<i>T</i><sub>g</sub>) from\nits bulk values, was directly proportional to chain rigidity. This\nrelationship between chain rigidity and enhanced segmental mobility\nwas further corroborated through molecular dynamics simulations. Thinner\nfilms exhibited a higher degree of crystallinity for all conjugated\npolymers, and a significant reduction of more than 50% in elastic\nmodulus was observed for films with approximately 20 nm thickness\ncompared to those of 105 nm thickness, particularly for highly rigid\nconjugated polymers. Interestingly, we found that the charge mobility\nremained independent of film thickness, with all samples demonstrating\ngood charge mobility regardless of the different film thicknesses\nfor devices measured here. Nanoconfined conjugated polymer thin films\nexhibited a combination of mechanical compliance and good charge carrier\nmobility properties, making them promising candidates for the next\ngeneration of flexible and portable organic electronics. From an engineering\nstandpoint, confinement could be an effective strategy to tailor the\ndynamics and mechanical properties without significant loss of electronic\nproperty.